This invention relates to an effective value measuring apparatus in which a triangular signal is superimposed on a signal to be measured and a mean value of the triangular signal is detected to obtain a square value of the signal to be measured, thereby measuring its effective value.
Various methods have heretofore been proposed for measuring an effective value of an alternating signal. One of the conventional methods that has been widely employed is to supply a signal to be measured to a heater and measure heat generation of the heater with a temperature sensitive element, such as a thermocouple or the like. But this method has the defects of large time lag due to the use of the heater, low accuracy owing to the use of the temperature sensitive element, susceptibility to the influence of the ambient temperature, etc.
Letting Vi and T represent a signal to be measured and its period, respectively, its effective value Ve is given by the following expression: ##EQU1## This equation (1) shows that the effective value Ve can be obtained by squaring the signal to be measured Vi and extracting the square root of a mean value of its one period. Accordingly, the effective value can be measured by achieving the operation by the employment of electrical circuits. In such a case, a square circuit, i.e. a multiplier is needed for the signal to be measured but this method encounters the problems that the multiplier is generally complicated and that a highly accurate multiplier is not easily available.
On the other hand, the ratio between the areas of similar triangles is proportional to the ratio between the squares of the heights of the triangles. There is proposed, for example, in a Japanese magazine, Trans. Inst. Electronics Comm. Engrs. Japan, Pt. C, Vol. 59, CNo. 1, 1976 "Digital Effective Value Voltmeter with Wide Operation Range," a method of measuring the effective value Ve by utilizing the above fact, that is superimposing the signal to be measured on a triangular signal, averaging the triangular signal above a zero level, i.e. obtaining an output corresponding to the area of the triangular signal, to obtain the sum of squared values of the levels of respective peak points of the signal to be measured and extracting the square root of the abovesaid sum. With this effective value voltmeter, since the root-mean-square value of the signal to be measured Vi is obtained by using the triangular wave, no multiplier is needed and no time lag occurs unlike in the case of employing the heater and, on top of that, the effective value can be measured with higher accuracy than is obtainable with the use of the temperature sensitive element. But a change in the amplitude of the triangular signal causes a change in the root-mean-square value of the signal to be measured. Further, the square root extracting circuit used becomes relatively large in scale, in particular, for greater operation accuracy, the circuit becomes large-scaled and expensive. Moreover, since the output to be ultimately obtained is in the form of frequency indicating the effective value, a counter is required for measuring the frequency, so that a time standard is needed and if its accuracy is low, a large error is resulted.
A first object of this invention is to provide an effective value measuring apparatus which employs a triangular signal for obtaining the effective value of a signal to be measured and in which no measurement error occurs even if the amplitude of the triangular signal varies.
A second object of this invention is to provide an effective value measuring apparatus which is capable of accurate measurement even if the level of a signal to be measured greatly varies.
A third object of this invention is to provide an effective value measuring apparatus which is capable of accurate measurement regardless of the frequency of a signal to be measured.
A fourth object of this invention is to provide an effective value measuring apparatus which is relatively small in circuit scale, capable of accurate measurement and inexpensive.
A fifth object of this invention is to provide an effective value measuring apparatus which provides a measured apparatus which provides a measured output in the form of a voltage or current value and requires no time standard.